1. Field of the Invention
This invention relates to dielectric ceramics. More particularly, it relates to dielectric ceramics which have an exceptionally high dielectric constant, which show a high unloaded Q even in the high-frequency region of the SHF (super high frequency) band, and which are hence suited for high-frequency applications.
2. Description of the Prior Art
It is generally required that the dielectric ceramic materials used for dielectric resonators and substrates in signal circuits for high-frequency regions, such as microwave and millimeter wave bands, should have a high dielectric constant and a high unloaded Q. The tendency toward higher frequency is increasingly marked in recent years in telecommunication systems, and satellite broadcasting using SHF band is now coming into its own. Under the circumstances, there has been a strong demand for low-loss dielectric materials having a far higher unloaded Q. To be more specific, there is a demand for dielectric materials which have a dielectric constant as low as 30 to 100 and show an unloaded Q of 6000 or greater in the SHF band.
Dielectric ceramic materials conventionally used for high-frequency applications are those of TiO.sub.2 system--for example, BaO-TiO.sub.2 type materials (Japanese Patent Application Laid-Open No.61668, 1982), ZrO.sub.2 -SnO.sub.2 TiO.sub.2 type materials (Japanese Patent Application Laid-Open No.38100, 1976) and MgO-CaO-La.sub.2 O.sub.3 -TiO.sub.2 type materials (Japanese Patent Application Laid-Open No.49964, 1975)--particularly those having the Perovskite structure. In addition, dielectric ceramic materials of the complex Perovskite structure, such as Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 (Japanese Patent Application Laid-Open No.35454, 1978) and Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 (Japanese Patent Application Laid-Open No.206003, 1983), have also been proposed in recent years. However, any of these fails to meet the requirements mentioned above because the unloaded Q in the SHF region under consideration (especially in the X band) is smaller than 5000, or the dielectric constant is less than 30.